Image sensor arrays are in widespread use in digital cameras, camcorders, and many other products. Complementary metal-oxide-semiconductor (CMOS) is low cost and versatile and, thus, has become the technology of choice for most of these arrays. Within CMOS itself, many types of devices intended for visible imaging applications are in use. Such devices can be tailored to large-format still cameras, standard video cameras, and compact “web cam” units, for example, all with varying degrees of size, cost, and performance.
CMOS imaging devices generally have what is known as an “electronic shutter,” which can function as a replacement for a mechanical shutter. An electronic shutter controls the duration of the exposure on an array of pixel sensors, allowing the system to adapt to lighting conditions. Although an electronic shutter increases overhead in the device design, for most applications the electronic shutter can be worth the advantage of avoiding a mechanical shutter.
A “rolling shutter” is one type of electronic shutter. The rolling shutter can be provided while incurring almost no costs and, therefore, is widely used. For the rolling shutter, an array is scanned one row at a time for readout. At each row read-out, the pixels in the row have been integrating photocurrent for a certain period of time, Tint, because the pixels have been reset by a separate scanning event Tint ago. A vertical scanner can control the separate scanning event. A drawback of the rolling shutter is that while the entire array has a uniform Tint, the actual start time and end time are different. For example, the top rows of the array may be exposed before the bottom rows of the array are exposed. Thus, horizontally moving objects might be distorted. While Tint can be made to have a short duration, the start time and end time might be separated by up to an entire frame period in some cases.
To overcome the drawbacks of a rolling shutter, an electronic “global shutter” can be used. For global shutter, all pixels start and stop integration at the same time, which can avoid the motion artifact of the rolling shutter and can approach the performance of a mechanical shutter. However, the global shutter has more overhead at the pixel, circuit, and system levels. While there are many ways of implementing electronic global shutter, in CMOS a common approach is to use a five transistor (5T) pixel, which is a compact type of shutter that can provide acceptable performance.
The above-described deficiencies of today's image sensor arrays are merely intended to provide an overview of some of the problems of conventional arrays, and are not intended to be exhaustive. Other problems with conventional arrays and corresponding benefits of the various non-limiting embodiments described herein may become further apparent upon review of the following description.